Combustion occurs by the initiation, propagation, and termination of oxygen free radical reactions. In combustion engines, boilers, etc., the free radical reactions between atmospheric oxygen and carbon-based fuels are typically initiated thermally. The reactions result in oxidation of carbon, forming carbon dioxide (CO2) and carbon monoxide (CO), when the predominant species is based on the ratio of carbon to oxygen present in the fuel/air mixture. Maximum efficiency in terms of energy output is favored when carbon dioxide is the end result, and this reaction is favored at high temperatures when there is sufficient oxygen present to complete the reaction.
Other elements are typically present in the fuel and in the air, and on reaction with oxygen free radicals during combustion, oxides of nitrogen (NOX), oxides of sulfur (SOX), water, etc., are formed. In cases when the amount of oxygen is insufficient to cause complete oxidation, unburned fuel can cause emission of particulate materials, soot, unburned hydrocarbons, etc. Particulates/soot, unburned hydrocarbons, CO, NOX, and SOX are considered unwanted byproducts of fuel combustion, and steps are taken post-combustion to reduce their presence in exhaust gases resulting from the combustion process. CO2 is a byproduct of combustion that is known to be environmentally damaging as well.
Noble metals of the Group VIII elements are known to be particularly efficacious for the purpose of forming oxygen free radicals to promote combustion. Other Group VIII metals such as iron, cobalt, and nickel are intermediate in terms of their ability to promote formation of oxygen free radicals but are suitable for the purpose of enhancing combustion efficiency. While combustion enhancement via catalysis is useful from an efficiency standpoint, from an emissions standpoint, improvements in combustion efficiency also cause a rise in unwanted NOX and SOX levels in exhaust gases. An improvement on the one hand has a concomitant adverse effect on the other.
In view of the above, a need remains for improved additives for combustible fuels that can reduce harmful emissions, improve combustion efficiency in exhaust gases, increase engine torque, increase engine horsepower, and generally improve fuel economy.